
Click the Columns button in the toolbar above the performance summary graph.
The column selection tool appears. 
Click Formula columns under the Available columns header.
A list of your existing Formula columns appears. 
Click + New column.
The popup with the settings for your new column appears. 
Name your column.
This is the name that will appear in the reporting table, so make sure it’s meaningful to all users who will see it. 
Add one or more columns to the formula:

In the Search columns and functions field, start typing an existing column name (or a word in the name).

When the column name appears, click it.
The names of existing formula columns start with c:, such as c:”Formula_column_name”.
A message appears that indicates whether the column is eligible for use in a bid strategy. 
The column will appear in the Enter equation box.
You can also enter a column name directly in the Enter equation box.
See the list of all available columns. 

In the Enter equation box, add the desired functions.
See the list of functions you can use in formula columns. 
Next to When do you want to apply this formula?, specify how summary rows in a reporting table aggregate data from the formula column, and whether executive reports that include the formula column report data from multiple advertisers:

Before totaling data: applies the formula to each summary row and then the results are totaled.
For example, for a formula column that divides keyword cost by the number of days in a report, this option divides the keyword’s cost by the number of days in each summary row and then adds up the results.
This option also enables executive reports to include data across multiple advertisers. That is, if an executive report is scoped to multiple advertisers and you add a formula column to a chart or table, the chart or table will apply the formula to each row and then sum the data from all advertisers in the report's scope.

After totaling data: sums the raw data in the individual rows and then applies the formula to the sum.
For example, for a formula column that divides keyword cost by the number of days in a report, this option sums the total cost first and then divides the total by the number of days.
With this option, the formula column cannot be used in executive reports that are scoped to multiple advertisers.

 Click Save.
 If an error occurs, click Select error to highlight the specific error, attempt to fix it, and click Save again.
 Once you have resolved all errors, the column will automatically appear in the list of selected columns. If you’re editing an existing column, you’ll need to manually add it.
To add the formula column to a reporting table:

Under Selected columns, drag and drop the new column to the desired location. If you’re editing an existing formula column, you’ll first need to manually drag and drop it to the selected columns.

Click Apply.
The new column appears in your reporting table.